Various methods have been devised for enabling the safe, continued operation of unpressurized or underpressurized vehicle tires with the intent of minimizing further damage to the uninflated tire and without compromising vehicle handling over a distance from the place where the tire lost its pressure to a place, such as a service station, where the tire can be repaired or replaced. Pneumatic tires designed for sustained operation under conditions of unpressurization or underpressurization are also called runflat tires, as they are capable of being driven in the uninflated or “flat” condition.
One approach to runflat tire design is described in U.S. Pat. No. 4,111,249, entitled the “Banded Tire,” in which a relatively thin annular band approximately as wide as the tread is deployed radially inward of the tread. The band behaves as a structural flexible-arch compression member allowing coacting tension loads in the sidewalls to act over a substantial portion of the circumference of the sidewalls during unpressurized operation. As described by European Patent No. EP 0 853 009 A2, banded tires have been fabricated with band materials made of steel, aluminum, titanium, and epoxy and thermoplastic composites with glass, Kevlar, and graphite fiber reinforcement. The common failure mode with the light, economical laminate band construction is interlaminar shear within the band's primary neutral bending axis.
Another approach to the design of runflat tires is to strengthen the sidewalls. For example, U.S. Pat. No. 5,368,082 ('082) discloses a low aspect runflat pneumatic radial ply tire, which employs special sidewall inserts to improve the stiffness and strength of the sidewalls permitting the tire to be driven while uninflated. Due to the large amounts of rubber required to stiffen the sidewall members, heat buildup due to flexure of the sidewalls is a major factor in tire failure, especially when the uninflated tire is operated for prolonged periods at high speeds.
The design of runflat tires relying primarily on thickened sidewalls is even more problematic when applied to high-aspect-ratio tires for large luxury touring sedans. These taller sidewalled tires, having aspect ratios in the 55% to 65% range or greater, means that the sidewall bending stresses are greater than that of the earlier low-aspect-ratio runflat tires disclosed in the '082 patent. Thus the sidewalls of high profile tires may require stiffening to the point of compromising ride characteristics. Luxury vehicle owners generally do not wish to sacrifice ride quality for runflat capability.
Thickened sidewalls also contribute to the problem of tread buckling during runflat operation. When the tire is uninflated, the vehicle load is transmitted from the sidewalls primarily to the edges of the tread footprint while the center of the footprint remains essentially unloaded. Moreover, as the thickened sidewalls bulge under the load of the vehicle weight, they tend to transmit bending stresses to the tread. The result is that the tread, in the central portion of its footprint, tends to buckle upward from the ground. This tread buckling reduces the area of tread in contact with the ground, resulting in compromised vehicle handling, as well as reduced runflat tread life. One approach to control tread buckling under runflat conditions is to increase the lateral and circumferential stiffness of the tread structure by incorporating wedge inserts in various locations beneath the tread, belts and carcass to control runflat tread buckling. An examples of a reinforced tread is described in PCT Patent Application PCT/US98/06004 filed 26 Mar. 1998.
U.S. Pat. No. 5,685,927, ('927) discloses a runflat tire comprising at least three annular bead cores located coaxially with respect to the axis of rotation. The first and second bead cores are located radially inwardly of each sidewall. At least one additional bead core is located under the tread radially inwardly of each pair of reinforcing belts and the radially outwardly of the first and second bead cores and a carcass reinforcing structure. In addition, '927 shows a reinforced sidewall construction that is substantially thinner than predecessor type runflat tires. This thin sidewall construction is made possible by the employment of the additional third bead. The additional bead core not only keeps the tread package unbuckled when the tire is operated under load and uninflated, it actually contributes to the load carrying capacity of the tire.
The design of an optimum runflat tire presents a complex challenge where multiple design criteria should be considered simultaneously. A runflat tire should provide a structure sufficient to support the vehicle weight without air pressure. The tread should resist buckling in the area of its contact with the road to provide adequate and consistent traction. In addition, the tire should provide a comfortable ride while either in the inflated or uninflated condition. Finally all these areas of performance should be addressed while minimizing the additional weight of the tire, the complexity of its construction and its cost.